1. Field of the Invention
The present invention relates to a digital broadcasting receiver adapted for digital broadcasting systems in which a synchronizing signal with fixed pattern is transmitted in a frame with each carrier subjected to phase modulation (PSK) and orthogonal frequency division multiplexing (OFDM).
2. Description of the Background Art
As shown in FIG. 12, in an orthogonal frequency division multiplexing (OFDM) digital broadcasting system with phase modulated (PSK) carriers, an OFDM signal in the RF frequency band inputted from the antenna 1 is amplified in the RF amplifier 2, multiplied by a signal outputted from the voltage-controlled oscillator 10 in the mixer 3 to be downconverted into an intermediate frequency (IF) signal, bandwidth-limited and amplified to an appropriate signal level in the IF amplifier 4, quadrature demodulated in the quadrature demodulator 5, and then the I (in-phase) signal component and the Q (quadrature) signal component in the baseband frequency band are inputted to the A/D converter 6. The signals of the I component and the Q component converted into digital data in the A/D converter 6 are inputted to the symbol selector 15 and partitioned for each symbol, inputted to the FFT processor 7 and converted into complex number data in the frequency domain, inputted to the differential demodulator 20 and differentially demodulated, inputted to the error corrector 8, and then the error-corrected data is outputted to the digital output terminal 9.
In the OFDM signal with phase-modulated carriers, if the frequency downconverted by the mixer 3 is ideal, phase data of the individual carriers outputted from the FFT processor 5 concentrate at some particular phase points (for example, the four points: 0, .pi./2, .pi., -.pi./2 in the quadrature PSK (QPSK)). When the frequency downconverted by the mixer 3 is shifted from an ideal value, phase data of the carriers outputted from the FFT processor 7 appear while being shifted from the original phase points.
Accordingly, the data outputted from the FFT processor 7 and differential-demodulated in the differential demodulator 20 are inputted to the phase error detector 31. The phase error detector 31 calculates a deviation of phase (a phase error) of each carrier from the original phase point and the phase error is inputted to the frequency deviation detector 14. The frequency deviation detector 14 controls the oscillation frequency of the voltage-controlled oscillator 10 to reduce the phase error and thus the center frequency of the IF signal downconverted by the mixer 3 approaches the ideal value. More specifically, in the case where each carrier is modulated by QPSK with the four phase points, 0, .pi./2, .pi., -.pi./2, values obtained by raising data of the carriers outputted from the FFT processor 7 to the fourth power ideally all provide phase of 0 (actually, multiples of 2.pi.). Accordingly, a summation of all results obtained by raising to the fourth power is regarded as a phase error. The frequency deviation detector 14 controls the oscillation frequency of the voltage-controlled oscillator 10 by using the phase error and the signal outputted from the voltage-control oscillator 10 is inputted to the mixer 3, thus determining the center frequency of the IF signal outputted from the mixer 3.
In the case of the QPSK, however, when the frequency downconverted by the mixer 3 is deviated from the ideal value, the phase data of each carrier outputted from the FFT processor 7 may appear while being deviated from the original phase point by about .pi./2. In this case, when data of each carrier outputted from the FFT processor 7 are raised to the fourth power, phases of these values are all approximately 0 (actually, multiples of 2.pi.). Then, the deviation of the frequency downconverted by the mixer 3 can not be corrected by controlling the oscillation frequency of the voltage-controlled oscillator 10 by using the value of summation of all the results. The same is true when phase data of each carrier is shifted by -.pi./2 or .+-..pi. from an original phase point. Similarly, a frequency deviation equal to or greater than the carrier frequency interval of OFDM can not be corrected, either.